Add a listen hash analogous to events

I've seen #1903 and I hope that I've addressed the concerns that @jashkenas raised. This seems to be something that would be welcomed and (to me) is a great addition to this awesome library.

Usage is as follows:

```javascript
listeners: {
  'event': 'event', // this.listenTo(this, 'event', ...);
  
  // this.listenTo(this['model'], 'change', ...);
  'change model': 'modelChange',
  'custom eventable': 'eventableCustom' // ...
},
```

A possible improvement is to allow multiple events as follows:

```javascript
listeners: {
  'add remove reset collection': 'stuffHappened',
},
```

If @jashkenas et all are accepting of allowing multiple events then I'll add it readily.

Let me know of any further thoughts or concerns. I'd love to have this part of backbone core.